A frequently employed technique for eliminating visual defects of the human eye—such as, for example, short-sightedness or long-sightedness or/and astigmatism—is so-called LASIK. LASIK stands for laser in-situ keratomileusis and designates a technique in which firstly a small disc (lamella) is cut free on the surface of the cornea, said disc being folded aside in order to expose the underlying tissue regions of the cornea. These exposed tissue regions are then treated in ablating manner by means of focused UV laser radiation, i.e. corneal material is resected in accordance with an ablation profile ascertained individually for the patient.
The surface disc of the cornea which is cut free is usually designated in specialist circles as a flap; it is not detached completely from the remaining cornea but is still connected to the remaining corneal tissue in a hinge region, commonly designated in specialist circles as a hinge. This enables a simple folding-away of the flap and, above all, a simple folding-back of the flap after the ablation. On account of the resection of material, after the flap has been folded back a changed shape of the anterior surface of the cornea appears. This results in a different refractive behaviour of the cornea and consequently of the overall system constituted by the eye. By suitable establishment of the ablation profile, it can be ensured that the visual defect is at least distinctly attenuated and at best is even almost completely eliminated.
Various procedures are known in the state of the art for the preparation of the flap. One procedure utilises a mechanical microkeratome, i.e. a microsurgical plane which cuts into the cornea with a cutting blade which is ordinarily driven in oscillating manner. Another procedure, which will be considered in more detail within the scope of the invention, utilises focused ultra-short-pulse laser radiation for the purpose of preparing the flap. Ordinarily, laser radiation with pulse durations within the femtosecond range, but at any rate within the low picosecond range, is employed in this case. For the placement of corneal incisions, the laser radiation that is used for this purpose must have a wavelength above about 300 nm, in order to enable a coupling of the radiant energy deep into the corneal tissue. LASIK treatments in which the flap is prepared by means of such ultra-short-pulse laser radiation are often designated as fs LASIK.
For the generation of incisions by means of focused laser radiation in transparent material (transparent to the laser radiation), the so-called laser-induced optical breakthrough is utilised by way of physical effect. This results in a photodisruption of the irradiated tissue in the region of the focus. By setting a plurality of such photodisruptions alongside one another, two-dimensional and three-dimensional incision figures can be realised in the cornea (and also in other tissue parts of the eye, which, however, will not be considered further here). The radiation parameters of the laser radiation may have been set in such a way that each individual laser pulse results in a photodisruption. Equally, it is conceivable to set the radiation parameters in such a way that a photodisruption occurs only after beaming several (at least two) laser pulses onto substantially the same point.
Especially in the case of the correction of a case of myopia (short-sightedness) by a LASIK treatment, the problem arises that after the ablation the flap can no longer fit optimally into the wound area (corneal bed). This is because for the purpose of correcting a case of myopia the most intensive resection of material commonly takes place in the centre of the ablatively machined optical zone. As a result of this, the radius of curvature of the optical zone decreases in comparison with the state before the ablation. This is accompanied by a diminution of the arc length of the optical zone measured along the surface. If the flap is now folded back onto the corneal bed, it may be that it does not fit perfectly snugly into the bed but that creases arise in the flap. This phenomenon, also designated as striae, may give rise to unpleasant impairments of the vision of the patient. For the purpose of eliminating the complications as a consequence of striae of the flap, one idea may be, for example, to heat the flap after folding it back onto the bed and to smooth it out. However, this constitutes an additional burdening of the patient by virtue of a further treatment step.